Corvette Carburetor Restoration - Technically Speaking

Carburetor Tech: Time for a Rebuild?

Q: I own a survivor '68 427 Corvette with a Holley carburetor. It has started leaking out of every orifice where there is a gasket. I have been running pump gas in the car, and I suppose the alcohol has gotten the best of it. I was thinking of rebuilding the carb myself, but a friend of mine suggested that the job might be too difficult for an amateur. (I'm not sure I trust him: He owns a Mopar.) So the million-dollar question is, who can I trust with the rebuild?

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Matt, Via the Internet

A: When it comes to carburetor restoration, my personal opinion is that you're better off having a specialist perform the job. Most local garages simply won't have the proper equipment or the skills to do an all-inclusive rebuild.

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Fortunately, there are several companies throughout the country performing this service. We decided to take a trip to one such outfit, Daytona Parts Company, and spend the day with owner Ron Hewitt. We'll take you through a Holley-carburetor restoration so you can see some of the differences between a do-it-yourself rebuild and one that is performed by a pro.

Hewitt gave us a tour of his facility, where every aspect of the restoration is performed. We found stacks of original engineering drawings that DPC's rebuilders refer to on each restoration; this ensures that every carburetor is as close to original as possible. Before any work begins, two or three close-up photos are taken of each carburetor from different angles so the carb can be easily identified. (As you can imagine, carburetors show up at the shop in a variety of conditions.)

The process starts with an inspection of the carburetor's overall condition; this helps the tech determine what's original and what's not. Remember, some of these carburetors are more than 50 years old. If your carb was ever used as a core and sent to a production rebuilder, it's possible that the parts were mismatched during the overhaul. A production rebuilder will normally tear down 10 to 15 like units at a time and run all of the components through a cleaner before the reassembly process begins. This can easily cause a mismatch.

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Good restoration shops will check all the part numbers found on the carburetor, including the main and secondary metering blocks, as well as the list and date codes. They will also inspect items that were changed through the production run, such as throttle shafts or float bowls, to ensure that the most-correct parts are put back on the carb. An inspection for any part numbers that may have been re-stamped is also performed. Hewitt finds several of these each year, especially on Carter AFB (Aluminum Four-Barrel) carbs.

Carburetors are precision parts, so an inspection of overall condition is important. Hewitt looks for metal fatigue, slightly stripped threads, hairline cracks, worn throttle-lever bushings, and pitting from oxidation, all of which can be a problem with older carbs. It's usually impossible to repair pitting during the re-plating process, as there is typically not enough material to grind it out. If you're looking for the correct carburetor for your car at a swap meet, remember to try to find a core unit that fits well, with no oxidation or pitting.

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A good rebuilder will contact you before teardown to let you know about any wrong parts or condition problems that were found during the inspection. Fortunately, most restorers have an abundance of extra parts they can draw from if a problem is found with your unit.

Now, let's address the teardown process. This where your carburetor is disassembled and separated into unlike alloys for correct cleaning in an acid bath, glass beader, or ultrasonic cleaner, depending on the material. After cleaning, most parts are cleaned again in a special vibrating tumbler using ceramic pellets. This is all in preparation for the correct original finishes to be applied before re-assembly. There were many finishes used on carburetors, including black oxide, various colors of cadmium, zinc, and more (Image A). These vary depending on what kind of carb is being rebuilt, the metal alloys involved, and the era it comes from.

One area that separates a specialized carburetor rebuilder from a do-it-yourselfer or local shop is the details. For example, when rebuilding the baseplate, Daytona Parts Company bores out each end of the plate where the throttle shaft runs and install a bronze bushing (Image B). The throttle shaft will also be nickel plated to return it to its original size (Image C). This is done due to the wear that occurs over years of use. When you accelerate, you're pulling the throttle shaft back toward you; over time this can cause the baseplate to become egg shaped and leak. New Teflon bushings are also installed, as necessary, on the throttle shaft during.

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When installing the throttle plates back into the baseplate, it's important to make sure that they move freely and that you can see a perfect “halo” around them (Image D). Once that's done, all of the throttle plate screws are installed with Loctite.

The primaries and secondaries will need to be synchronized by bending the connector link with a special tool (Image E).

Vehicle backfires often cause power-valve failures. These typically result when a vehicle is stored, and the bowls on the carburetor become dry. When the engine is cranked, it “pops” back through the carb. Daytona Parts Company helps reduce the chances of this happening by drilling a relief hole in your original baseplate (Image F) and installing an anti-backfire check valve (Image G).

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A professional rebuilder will remove all of the plugs from the secondary metering block so these areas can be cleaned of sediment (Image H).

Another concern with older Holley carburetors is that the float bowls may have become worn or stripped where the needle-and-seat adjustment is located. A professional rebuilder will bore out the float bowls where the float-adjustment screw is located and install a brass bushing so the customer can retain the original bowl (Image I).

Tension is removed from the secondary diaphragm spring by placing the diaphragm housing in a cup (Image J). This prevents the diaphragm, which is made from rubber, from becoming distorted upon assembly. Four different diaphragm-shaft lengths were used on Holley carburetors, so care is taken to ensure that the correct one is installed. If the diaphragm is too short, the secondaries they will not close correctly, and may even stay open all of the time. If the diaphragm is too long, the secondaries may never open at all.

After installing the secondary diaphragm assembly onto the main body, it's pressure tested to ensure that the secondaries open and close correctly (Image K).

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The accelerator pump will need to be adjusted to specification, to ensure that the proper amount of fuel is delivered under acceleration (Image L).

The choke assembly has several adjustments, including air gap and choke unload. It's important that your choke is working correctly for several reasons, including getting the rpm up to approximately 1,800 to achieve oil pressure as soon as possible. An incorrect rpm choke setting can cause an overly rich fuel mixture on cold startup, which can “wash down” the cylinder walls with fuel and foul the plugs.

We asked Ron Hewitt for his recommendations on keeping our freshly restored carburetor working efficiently for as long as possible. He offered the following suggestions:

- Avoid alcohol-laced fuel
- Keep as little fuel in the tank as necessary when the vehicle is in
- Use a fuel with the lowest octane level your car will allow
- Start your engine at least once a week and let it warm up to operating temperature.

Like the factory before him, Hewitt also warned against using lubricants on the carburetor. Lubricants attract dirt, which can increase wear on moving parts.

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It's clear that there are a lot of adjustments and modifications that need to be made during the rebuilding process. While a local shop may have the ability to rebuild certain parts of the carburetor, it likely lacks the skills and the tools needed to perform a proper restoration. My advice is, if you need your carburetor rebuilt, send it to a professional who specializes in such work. You may well find that the cost is the same as letting the corner garage do the job.

If you have any questions, you can always give the guys at Daytona Parts Company a call at (386) 427-7108, or visit online at www.daytonaparts.com. Thanks for your question, and good luck.

Q: I'm trying to figure out the exact date on my 1968 Holley carburetor. I'm confused by the letter “B” in the month-of-manufacture position, as I always thought this code was a number. Could it be a stamping mistake? I've enclosed a photo of the air horn with the stamping.

Andy, Via the Internet

A: Many Corvette parts were coded to show when they were cast or assembled. Holley carburetors have three distinct stampings embossed into the driver side, front vertical surface of the air horn.

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The top stamping is a seven-digit GM part number that may be followed by one or two letters. Below the GM part number is the Holley part, or “list,” number. The stamping reads “LIST,” followed by four numbers. Some Holley carburetors have an additional number, letter, or combination of numbers and letters.

Below the Holley list number is the three-digit date code, which can be deciphered as follows:

The first number designates the year of manufacture, represented as the last digit of the calendar year. For your application, 8 = 1968.

The second number or letter is the month of manufacture: 1 through 0 for January through October, and then the letters A and B for November and December. For your application, B = December.

The third number represents the week of the month the carburetor was assembled (first through sixth). For your application, 5 = fifth week of the month.
Some original-equipment Holley carburetors may be coded using a four-digit date code. These are usually service carburetors or replacement parts. On these units the first three digits represent the Julian calendar date (001 through 365, or 366 for leap year). The last digit of the date code represents the year of manufacture, represented as the last digit of the calendar year.

The system can seem daunting at first, but with a little work, you should be deciphering Holley date codes with a minimum of fuss.

Check out the March 2014 edition of Technically Speaking! James Berry helps our readers with any tech questions. This time, he helps Kevin out with a C4 PKE system issue with the battery and programming. Read along for more! » Read More